Defining the processes needed to operate a new service is part of:

ITIL's Service Design stage covers five aspects, one of which is explicitly 'Design the processes,' which involves identifying, documenting, and planning all management and operational processes required to support the new service. Addressing process design here ensures that all necessary processes are ready before the service enters Service Transition and eventually Service Operation.

Service Strategy is concerned with defining service value, market spaces, and strategic offerings - not with the detailed design of operational processes.

Service Transition plans and prepares for deployment using processes that have already been designed; it does not define them.

Service Operation executes defined processes day-to-day but is not the stage where processes are formally designed and specified for a new service.